By Gary Sliworsky
Ag rep, Emo
The growth of most pastures is limited more by the lack of nitrogen than any other essential element.
It is a major constituent of plant proteins, so it is necessary for plant growth and metabolism.
Plants lacking nitrogen are slow-growing, small, and light green in colour. Pastures that are nitrogen deficient are unproductive, slow to recover after being grazed, and offer a poorer quality feed to grazing animals.
The ideal supply of nitrogen to a pasture is from the fixation of nitrogen in the air by bacteria living in nodules on the roots of legumes.
This source of nitrogen is continuous throughout the growing season, provides all the nitrogen required by the legumes and some for the grasses, leaves overwintering nitrogen in a non-leachable organic form, and is low priced.
Pastures with a legume content greater than 50 percent generally require no additional nitrogen for good yields.
However, a small amount (25 pounds/acre) of nitrogen can be used on legume-grass pastures in the spring in order to get the grass off to a fast start and provide early grazing.
As the grass content in a pasture increases, so does the requirement for added nitrogen.
To produce the same amount of forage from a predominantly grass pasture as from a legume-grass pasture requires the addition of nitrogen in a readily available form such as manure or commercial fertilizer.
High yields from grass pastures are obtained more efficiently by using several small applications of nitrogen rather than one large one.
Frequent applications also produce a more uniform distribution of yield over the grazing season.
Even in seasons with a dry period, production is better when nitrogen is applied two or three times rather than in one large application.
Nitrate poisoning is a potential hazard when grasses receive nitrogen prior to a period of poor growth and precautions should be taken.
The minimum amount of nitrogen needed to produce yield response is 45 pounds/acre. Individual applications on pastures should not exceed about 65 pounds/acre.
Above this rate, the chances of nitrate poisoning increases, especially if the application is made in the summer of fall.
Fast-growing plants make the best use of applied nitrogen. Demand for nitrogen is highest in the spring and immediately after grazing.
In pasture rotations with rest periods of less than 30 days, smaller nitrogen applications should be used since the grass does not have enough time to efficiently make use of large amounts of nitrogen.